1. Field of the Invention
The present invention relates in general to automatic transmissions, and more particularly, to control systems for controlling automotive automatic transmissions.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional control system for an automotive automatic transmission will be outlined, which is disclosed in Japanese Patent First Provisional Publication 2-256953.
The control system shown by the publication is so constructed that when a brake pedal is depressed by a degree greater than a reference value, the transmission is forced to effect a downshift. That is, when, with the vehicle moving at a speed lower than a given value and with the throttle valve being fully closed, the brake pedal is depressed beyond the reference degree, a so-called "downshift instruction signal" is applied to a downshift actuating means of the transmission to achieve the downshift of the same. With this, engine braking is obtained at the time when the brake pedal is depressed under the above-mentioned condition.
In the control system, however, the reference value of the pedal depression for achieving the downshift is fixed. This means that it sometimes occurs that the downshift does not take place even when the vehicle is under a condition wherein the downshift is really needed. That is, when the depression degree of the brake pedal does not reach the reference value, the depression does not induce a downshift even when a change has been carried out, for example, from a D-range wherein an overdrive is permitted to another D-range wherein the overdrive is inhibited (that is, when the overdrive inhibiting switch has been operated ON). Of course, in this case, the downshift actuating means does not operate and thus the engine braking is not obtained. This deteriorates the drivablity of the vehicle.